A gas turbine engine includes one or more turbine blade rows disposed downstream of a combustor which extracts energy from combustion gases generated by the combustor. Disposed radially outwardly of the rotor blade tips may be a stator shroud which is spaced from the blade tips to provide a relatively small clearance between the blade tips and shroud for reducing leakage of the combustion gases over the blade tips during operation. Each of the rotor blades includes conventionally known pressure and suction sides which are preferentially aerodynamically contoured for extracting as much energy as possible from the combustion gases flowing over the rotor blades. The pressure and suction sides extend to the blade tip and are disposed as close as possible to the stator shroud for maximizing the amount of energy extracted from the combustion gases. The clearance, however, between the blade tips and the stator shroud must nevertheless be adequate to minimize the occurrence of blade tip rubs during operation, which may damage the blade tips.
Un-shrouded blades use a squealer tip to reduce hot gas leakage over the blade tip and reduce performance penalties. Such a tip design typically requires ribs, generally a pressure side rib and a suction side rib, to protrude from the blade tip floor. These ribs are relatively thin, which makes them difficult to cool effectively through conduction. Turbine blade tips and associated ribs, moreover, are exposed to the very high temperatures of combustion gasses flowing over their outside surfaces. These high temperatures and low cooling effectiveness lead to durability issues on the tip ribs and the potential for blade fallout at the end of the blade's life interval. Any tip ribs that suffer oxidation or cracks beyond the squealer floor will render a blade irreparable regardless of the overall airfoil condition.
Whether shrouded or un-shrouded, turbine rotor blades are typically hollow for channeling cooling air through the interior of the blade. This cooling air is provided from a conventional compressor of the gas turbine engine to cool the blades from the heat flux generated by the combustion gases flowing over the blades. The tip, or tip cap, portion of the blades is particularly susceptible to the damaging effects of the hot combustion gases and must be suitably cooled for reducing blade tip distress in the form of oxidation and thermal fatigue during operation. As the blade tip erodes during operation due to the blade tip distress, the pressure and/or suction sides of the blade are adversely affected, which decreases the aerodynamic efficiency of the blade used for extracting energy from the combustion gases. In addition, such erosion of the blade tip also increases the clearance between the blade tip and the stator shroud, which allows more of the combustion gases to leak over the blade tip, and, therefore, extraction of the energy therefrom is lost which also decreases aerodynamic efficiency.
Numerous conventional blade tip cap designs exist for maintaining the proper pressure and suction side flow surfaces of the blade at the tip cap as well as providing minimum clearances with the stator shroud. Numerous cooling configurations also exist for cooling the blade tips or blade tip caps for meeting life requirements of the blades without undesirable erosion thereof. Conventional design practice makes use of a tip shelf recess or an L-shaped trough defined by the tip shelf and a first tip wall disposed on the pressure side of the blade. The tip shelf may offer the advantage of providing a discontinuity on the airfoil pressure side of the blade tip, causing combustion gasses to separate from the surface of the blade tip, which may decreases the heat transfer capability of the hot gasses to the blade tip, and therefore may decrease the heat flux into the blade tip. Conventional design practice also makes use of straight round holes through the tip shelf for passing cooling gas from the hollow blade interior to the tip shelf and pressure side rib, with resultant tip cooling due to convective and film effects. The tip shelf recess provides a region for the cooling air exiting the interior of the blade to accumulate, thereby providing a film blanket of cooling air between the hot combustion gasses and the blade tip, thereby further cooling the blade tip.
Another approach to cooling the blade tip is to increase the total number of straight round cooling holes in the tip shelf to increase the total cooling flow and decrease the space available for hot gas to interact with the surface. Since cooling of the blade, including the blade tip, uses a portion of the compressed air from the gas turbine compressor, however, that air is unavailable for combustion in the combustor of the engine which decreases the overall efficiency of the gas turbine engine. Accordingly, cooling of the blade, including the blade tip, should be accomplished with as little compressed air as possible to minimize the loss in gas turbine engine efficiency.
Still another approach involves creating channels or indentations in the pressure side rib to direct cooling flow from the pressure side tip holes over the rim at desired locations to better cover the surface.
Yet another approach is to thicken the pressure side rim and drill cooling holes through the center and exit at the rim top face. It would be desirable to provide tip shelf cooling holes that are economical to install, provide an acceptable flow of cooling air over the blade tip shelf, and provide an improved film blanket of cooling air spread across the tip shelf, thereby better protecting the blade tip from hot combustion gasses.